Platinum complexes, the prototype being cisplatin (Cis-Pt), have been widely used as active anticancer agents (Ardizzoni et al., 1999; Nitiss, 2002) in a variety of human tumors, including testicular, ovarian, bladder carcinoma, head and neck, and non-small cell lung cancers. The outcome of treatments with cisplatin and other platinum-containing complexes has been strongly linked to their alkylating effects on DNA. However, the potential impact of platinum-complex-based therapy on cellular signaling and the therapeutic importance of such interactions have yet to be explored. Reports show that cisplatin induces activation of members of the mitogen-activated protein kinase (MAPK) pathways (Persons et al., 1999; Sanchez-Perez et al., 1998), which may influence drug-induced apoptosis.
Cellular responses to growth factors and cytokines are characterized by activation of the Signal Transducer and Activator of Transcription (STAT) family of cytoplasmic transcription factors (Darnell, 1997; Darnell et al., 1994; Schindler et al., 1995; Stark et al., 1998; Smithgall et al., 2000; Akira, 2000; Hirano et al., 2000; Bromberg et al., 1996; Fukada et al., 1996; Kotenko et al., 2000). STATs are activated at a very early stage involving protein tyrosine kinase phosphorylation of tyrosine associated with growth factor receptors, receptor-associated Janus kinase (Jaks) or Src kinase families. This in turn induces phosphotyrosine (pTyr)-SH2 interactions between two STAT monomers in the formation of dimers, translocation to the nucleus, and binding to specific DNA response elements regulating gene expression essential for cell proliferation, differentiation, development and survival.
Normal STAT activation is tightly-regulated and has a short duration, which is in keeping with normal cellular requirements for mounting a response to external stimuli. However, persistent activation of specific STAT proteins, particularly Stat3 and Stat5, occurs with high frequency in some tumors and has a causal role in malignant transformation by promoting growth and survival of transformed and tumorous cells, including those breast, prostate, head and neck squamous carcinoma cells, lymphomas and leukemias (Bromberg et al., 1999; Turkson et al., 1998; Bromberg et al., 1998; Catlett-Falcone et al, 1999a; Garcia et al, 2001; Grandis et al, 2000; Grandis et al., 1998; Nielsen et al., 1997; Nielsen et al, 1999; Epling-Burnette et al, 2001; reviewed in Bowman et al, 2000; Turkson et al, 2000; Song et al, 2000; Coffer et al, 2000; Lin et al, 2000; Catleff-Falcone et al, 1999b; Garcia et al, 1998). Of clinical importance, blockade of aberrant Stat3 signaling in malignant cells and whole tumors that contain them induces apoptosis and tumor regression.